1. Field of the Invention
The present invention relates generally to communications systems, and more specifically, to a digital satellite communications modulation and detection scheme for multiple-carrier link operation.
2. Background of the Invention
Digital communication systems are presently prevalent in both voice and data communications equipment. One such application is in satellite data link communications equipment, where minimum spectral efficiencies are required for uplink/downlink equipment designed to meet specific standards. In order to meet high datarate requirements on a band-limited channel such as a satellite connection, a minimum spectral efficiency must be met in conjunction with a low symbol error probability requirement. The symbol error probability is dictated by the energy per transmitted bit versus the noise spectral density (Ebt/No). System requirements for gain and noise level that determine Ebt/No are dictated by maintaining minimum energy and maximum noise levels at each link in the communications chain.
The modulation schemes employed in the above-described communications systems have typically been quadrature amplitude modulation (QAM) or quadrature phase-shift keying (QPSK), providing a band-limited signal with a high spectral efficiency. Each phase state (for QPSK) or amplitude-phase state (for QAM) is mapped to a particular value for an interval in the signal (often referred to as a “chip”). The particular phase/amplitude states used to represent information collectively form a “constellation”, which is so-called because of the shape defined by a phase-amplitude diagram of the particular modulation states. Selection of the constellation and mapping of multiple chip sequences to symbols provides for rejection of disallowed combinations or sequences, lowering the symbol error probability of the link by increasing the effective Ebt/No.
For satellite systems that operate the output amplifiers near saturation, as in a single-carrier system, phase-shift-keying (PSK) has traditionally been a modulation of choice, as PSK detection is not degraded for amplitude compression of the signals due to non-linearity introduced in a high power amplifier (HPA). However, in multi-band satellite links, where multiple carriers carry multiple data streams, non-linear operation of the HPA is avoided, as signal amplitude distortion and inter-channel interference is introduced by any non-linearity in the link.
Selection of a symbol rate with respect to a known bandwidth generally dictates a spectral efficiency and traditionally a particular modulation type. However, for PSK, as the number of states per chip are increased by increasing modulation order, the ability to distinguish between states decreases and the ability to determine a unique carrier phase reference without cycle slip deteriorates. A transition from QPSK, which provides a maximum spectral efficiency of 2 bits/s/Hz to a system in which a 3 bit/s/Hz requirement may be met, requires (for PSK) an 8 state system known as 8PSK.
Therefore, it would be desirable to provide a communications link providing a spectral efficiency of 3 bits/s/Hz that outperforms 8PSK so that improved link performance is obtained. It would further be desirable to improve carrier detection performance for a selected constellation.